1. Field
The invention is related to fusion splicing of optical fibers, and more particularly to a method of calibrating the arc of a fusion splicer.
2. Related Art
Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as strong as the fiber itself. The source of heat is usually an electric arc, but can also be a laser, or a gas flame, or a tungsten filament through which current is passed.
A basic fusion splicing apparatus consists of two fixtures on which the fibers are mounted and two electrodes. The fibers are placed into the apparatus, aligned, and then fused together. The development of automated fusion-splicing machines have made electric arc fusion (arc fusion) one of the most popular splicing techniques in commercial applications. Examples of fusions splicers include Fujikura model nos. FSM-45F, FSM-PM, FSM100M and FSM100P.
The gap between an electrode pair can be adjustable either manually or automatically driven with motors. See for example FIGS. 1A and 1B. FIG. 1A shows a wide gap between an electrode pair and FIG. 1B shows a narrow gap between an electrode pair. When the gap of electrode is larger, the arc power/current would be higher and arc shape would be wider. By controlling the gap, the heating temperature on the fiber will also be adjusted. Since there are tolerances in the mechanical and electrical components of the splicers, the heat generated by the arc may vary even when the power/current settings are the same. In addition, atmospheric conditions such as temperature, humidity, and pressure are constantly changing, which creates variability in the arc temperature. Therefore, it is necessary to calibrate the splicers in order to compensate for these discrepancies, because if possible, it is advantageous to operate a plurality of splicers at the same settings.
It is also necessary to take into account the diameter of the fiber being spliced when adjusting the gap between the electrodes that produce the arc. For example, when the cladding diameter is over 250 μm, the electrode gap should be set to a “wide” setting (e.g., 3 mm). FIG. 1A shows the electrodes pushed outward to enlarge the gap. On the other hand, when the cladding diameter is less than 250 μm, the electrode gap should be set to a “narrow” setting (e.g., 1 mm). FIG. 1B shows the electrodes pushed inward to shorten the gap.
When the fibers with different fiber cladding diameters are spliced, different arc power/current settings are required. Examples of typical power/current settings are shown in FIG. 2. For example, if a pair of fibers with 125 μm cladding diameters are to be spliced, a narrow electrode gap (e.g., 1 mm) is enough to supply an appropriate amount of heat. But for a fiber pair with 400 μm cladding diameters, a 3 mm wide electrode gap has to be used to provide enough heat to melt the fiber. In the case a 125 μm cladding diameter spliced to 400 μm cladding diameter, the wide electrode gap also needs to be used. Note that “250-125” in the table means that a 250 μm cladding diameter is being fused to a 125 μm cladding diameter.
There are two types of traditional arc calibration methods. One is the melt-back method with melt-back distance measured at axis (center) of fibers. When using this method, it is necessary to melt long portions of the fiber, which in turn causes large amounts of SiO2 to be deposited on the electrode tips, which changes the arc discharge condition. This method is still used in a number of field splicers which splices 125 μm cladding diameter telecommunication fibers. This melt-back method is shown in FIGS. 4A to 4D. FIG. 4A shows the fibers before melt-back. First, the fibers are heated so that the corners of the fiber “melt-back.” FIG. 4B shows the fibers during melt-back. FIG. 4C shows the fibers after melt-back. FIG. 4D shows the measured fibers melt-back value shown on a display. The melt-back is measured at the closest points of two fibers. Since the fibers melt-back a lot when heated by a strong arc, they normally form a shape like match sticks. Therefore, the melt-back is normally measured at center of fiber axes. The measured melt-back amount is then compared to the melt-back amount for the particular fiber in a table, such as shown in FIG. 3. If the measured melt-back amount differs from the amount in the table, the power is adjusted accordingly and another melt-back is performed. This process is repeated until the measured melt-back is the same as the table value. This can be a very time-consuming process.
The second conventional arc calibration method splices the fiber with axis offset (see FIGS. 5A and 5B). After splicing, the distance of the offset changed from the original offset alignment due to the surface-tension pulling-back force during the arcing is measured. The higher the fiber temperature is, the higher distance of offset change is. This process has to be performed several times by re-arcing and re-measuring to get an average value (see FIGS. 5C to 5E). This method is also been used in many splicers for 125 μm cladding diameter fibers only. A problem with this method is that the pulling back distance depends not only on the fiber temperature, but also on the fiber cleave angle, fiber types, etc. You also need to splice the fiber first. For an unknown fiber type, e.g., 300 μm cladding diameter fiber, it is impossible to splice it first without knowing the correct power/current for the arc. Therefore, this method only works well with a certain fiber type (such as Corning fiber SMF28 of 125 μm cladding diameter) and does not work with other fibers, such as 400 μm large diameter fiber.
When splicing fibers with a narrow electrode gap (1 mm) at lower power (<100 bit), the offset arc calibration is typically run everyday. With the arc calibration, a set of Arc Power (AP) compensation coefficients will be updated to make arc power equal among different splicers and under different electrode conditions. The arc calibration will correctly compensate the arc power/current up to 100 bit with narrow electrode gap.
However, the offset arc calibration works only for 1 mm electrode gap, arc power<100 bit and fiber outside diameter<250 μm. In addition, there are other difficulties with the offset arc calibration method. Sometime the operator needs to repeat five or ten splicing processes for one successful arc calibration.